Edible oils are those extracted from plant seeds, such as soybean, cottonseed, peanut, sunflower, corn, etc., and processed for human consumption. They are usually found as ingredients in foodstuffs such as bread, pastries, margarines, salad dressings and candies. Today, they are consistently used as the medium to fry foods. In practically all cases, these oils undergo the process of partial catalytic hydrogenation to prepare them for use in their intended product. Hydrogenation alters the molecular structure within the fatty ester chains of the oil triglyceride molecule by reducing the number of double carbon bonds that naturally occur.
Partial hydrogenation affects edible oils in two ways. First, it stabilizes the oil; it extends the time that the oil's flavor and smell are acceptable as a human food. Second, it converts the naturally occurring oils into forms that have melting and handling characteristics demanded by the application; the more an edible oil is hydrogenated, the higher its melting point becomes. For example, oils used in salad dressings must not coagulate on cold lettuce; margarine should remain firm on the breakfast table, yet melt on pancakes and biscuits; shortening should remain firm in the pie crust or cake batter, but quickly melt in the oven.
The basic batch hydrogenation process in use today has changed little since it was invented nearly a century ago. A closed vessel (converter) is filled with approximately 58,000 pounds of oil. The oil is heated to approximately 175.degree. C. with steam passing through internal coils. Then roughly 10.5 pounds of nickel powder is added as a catalyst; hydrogen gas is introduced continuously into the bottom of the converter and the hydrogen is vigorously mixed into the oil and nickel powder by an agitator. The ensuing hydrogenation reaction is exothermic and is controlled by cooling water passing through the original steam coils. The converter operates at a pressure of 20 to 30 psi while still releasing excess hydrogen and processing effluents from the top of the converter. Hydrogenation is arrested immediately upon cessation of agitation which is normally done several times during the process. Processing times vary from 15 to 60 minutes depending on the desired degree of hydrogenation.
Although it is not a part of the actual hydrogenation process, about 30 pounds of diatomaceous earth are also added into the converter vessel along with the nickel catalyst. The diatomaceous earth is used as a filtering aid to remove the nickel from the oil at the termination of the process. The finished batch of oil is placed in a holding tank and continuously circulated through wire filter screens to cleanse the oil of the nickel catalyst. The diatomaceous earth is trapped on the wire screen filters, in effect becoming the filter itself. The nickel powder is in turn filtered out of the oil by this diatomaceous earth "cake." The "cake" is reclaimed off of the filter screens and the nickel recycled. All in all during the actual hydrogenation process, there is a 0.072% mixture (by weight) of nickel catalyst and diatomaceous earth present in the oil.
There are several ways of accurately determining the amount of hydrogenation of edible oils. One is the Wijs method, recommended by the American Oil Chemists, Society (AOCS). A large, measured amount of iodine monochloride reagent is added to a specific quantity of partially hydrogenated oil. The reagent "saturates" the oil by eliminating any double carbon bonds remaining in the various esters. The excess reagent is titrated with sodium thiosulfate, using starch as an indicator. The amount of iodine reagent lost is called the iodine value, a number that decreases with increased hydrogenation. This method, though accurate, is a laboratory procedure requiring 30 to 45 minutes to perform.
Another technique of determining the degree of partial hydrogenation is to measure the oil's refractive index (RI). There is a direct, linear relation between RI and iodine value. RI linearly decreases with increased hydrogenation. An example is the relationship between RI and iodine value (I) of cottonseed, peanut, soybean, and linseed oils. It has the form EQU n.sub.D.sup.40 =1.4515+(0.000117)I (1)
where n.sub.D.sup.40 is the RI at a wavelength of 589.3 nm and at 40.degree. C. (See Bailey, A. E., Bailey's Industrial Oil and Fat Products, John Wiley & Sons, Inc., Vol. 2, 1982, pp. 40, 220). The AOCS Method Cc 7-25, "Refractive Index," is an accepted industry standard in determining the degree of edible oil hydrogenation. The technique is quick and can be performed in the process control room.
Several established designs of process refractometers are in common use today which depend upon light transmission through the oil in order to measure its RI. With any of these units, a reading can be made in a matter of minutes and a correlation of RI to iodine value read from a chart. The handicap with this method is that these refractometers operate on light transmission through the oil sample, and with nickel catalyst and diatomaceous earth in the oil, the transmitted light is diffused, making the refractometer response difficult to interpret. Therefore, an oil sample withdrawn from a batch of oil being processed must first be manually filtered before analysis. While the sample is being withdrawn, filtered and analyzed, the hydrogenation process must come to a standstill which sometimes means a delay of 10 to 20 minutes. It is common to have two or three test samples taken during the processing of an oil batch. Not only is this a worrisome delay to the oil processor, but also the accuracy and repeatability of the refractometer reading is subject to operator skill. It would be desirable to have an on-line refractometer that will perform accurately in the presence of catalyst and diatomaceous earth.
One type of refractometer that does not require light transmission through the process liquid, and hence is unaffected by the presence of catalyst and diatomaceous earth, is a critical angle refractometer. (See Maley, L. E., "Refractometers," Journal of Chemical Education, vol. 45, no. 6, 1968, pp. A467-A485.) Such a device consists of a glass prism or plate with a flat side mounted over a small opening in a pipe or tank wall. The back side of the glass (RI=n.sub.g) is in direct contact with the process liquid (RI=n.sub.1). Using Snell's law, a critical angle, .theta..sub.c, for light incident at the glass-liquid interface is selected according to ##EQU1## such that light is reflected back to a sensor when EQU n.sub.1 .ltoreq.n.sub.g sin .theta..sub.c ( 3)
While the operation of this device is not affected by the turbidity of the process liquid, the device is only a fixed comparator and is thus unsuitable for continuous measurement of refractive index values. Also, n.sub.g must be greater than n.sub.1 for there to be a critical angle.
Another type of refractometer which does not require light to pass through the process liquid, but offers a means of continuously measuring RI values, is a fiber optic refractometer. The efficiency with which optical fibers transmit light is determined by the disparity of RI that exists between the core and cladding materials. It follows that such a device could be used as a refractometer if the process liquid of interest became the "cladding" about a glass core. By measuring the efficiency at which such a fiber transmitted light energy, the RI of the liquid cladding could be determined. The concept of attenuated total reflectance (ATR) forms this major category of fiber optic refractometers. (See Kapany, N. S., and J. N. Pike, "Fiber Optics, part IV, a photorefractometer," Journal of the Optical Society of America, vol. 47, no. 12, 1957, pp. 1109-1117.) In these instruments, a conical beam of light with a uniform intensity, I watts/steradian, excites a glass rod or fiber (RI=n.sub.g). The transmitted light is then measured by a photosensitive device. The following equations define the numerical aperture NA, and transmitted light power, Pt ##EQU2## By measuring Pt, continuous values of n.sub.1 may be calculated provided n.sub.g and I are known.
In order for the device to work, n.sub.1 must be less than n.sub.g. To overcome the problem of the refractometer losing all resolution when n.sub.1 .gtoreq.n.sub.g, glass rods or fibers of a higher RI must be substituted.
A variation of the ATR fiber optic refractometer uses a laser beam incident on the end of the glass rod. (See David, et al., "Design, development and performance of a fiber optics refractometer: Application to HPLC," Review of Scientific Instruments, vol. 47, no. 9, 1976, pp. 989-997; also, U.S. Pat. No. 3,999,857, J. D. David, D. A. Shaw & H. C. Tucker, "Refractive Index Detector," 1976.) The beam angle into the rod is adjusted via a mirror moved by a micrometer until the edge of the "cone of acceptance" (i.e., the numerical aperture or NA) is found. Multiple reflections of the light propagating down the fiber make the transition very sharp. The micrometer reading correlates to the NA, and n.sub.1 can be calculated from equation (4a). The instrument locates the sharp light transition at the edge of the NA, but its output drops to a low, constant level once the incident beam angle exceeds the NA.
A fiber optic refractometer using Fresnel's equations has also been designed. (See Meyer, M. S., and G. L. Eesley, "Optical Fiber Refractometer," Review of Scientific Instruments, vol. 58, no. 11, 1987, pp. 2047-2048.) Monochromatic light is transmitted down a single mode fiber and reflects off the far end of the fiber, immersed in the process liquid. The core (RI=n.sub.co) at that end of the fiber is polished smooth, perpendicular to the fiber axis. Fresnel reflections from the core/liquid dielectric interface are transmitted back through the fiber to a photosensor. For light perpendicularly incident to such an interface, the Fresnel's reflection is ##EQU3## The reflected light is correlated to equation (5), and the refractometer operates for values of n.sub.1 either greater than or less than n.sub.co. There is a reflection null at n.sub.co =n.sub.1. However, due to the quadratic dependence of the reflectance, it is impossible to tell which side of the reflection null the refractometer is operating on. The solution is to use two fibers of different RI and observe their relative outputs.
Fiber optic refractometers using bent fibers have also been developed. (See Golunski, W., et al., "Optical fiber refractometer for liquid refractive index measurement," Proceedings of the SPIE--Optical Fibers and Their Applications V, vol. 1085, 1990, pp. 473-475.) Bending the fiber effectively decreases n.sub.co, reducing the NA of equation (4a). Consequently, the fiber's transmitted power, Pt, of equation (4b) is also reduced. One paper, using a stripped, step index quartz fiber, describes how the range of such a refractometer's operation (analog range of Pt) can be varied by controlling the radius of the fiber bend; the smaller the radius, the lower the RI sensing range becomes. (See Harmer, et al., "Optical fibre refractometer using attenuation of cladding modes," Proceedings of First International Conference on Optical Fibre Sensors, Electronics Div., Institute of Electrical Engineers, 1983, pp. 104-108.) Such a refractometer is only functional when the liquid RI is less than the effective n.sub.co.
Previous fiber optic refractometers, as described above, suffer from several shortcomings. First, analog optical responses are subject to variations in light source intensity and uniformity as well as light sensor sensitivity, liquid turbidity, operating temperatures and the bending of the optical fiber. Second, most responses of Pt versus liquid RI are nonlinear and must be compared to a complicated algorithm in order to maintain high accuracy. Third, there is no compensation for RI dispersion due to the light source wavelength. Fourth, mechanical measurement devices are affected by misalignment.